You need to use the Administrator's Console.
Under the global settings select the MTA tab.
Then there is a field called Maximum message size (kb). By default it is set to 10000 (10M). Change this value.

You need to use the Administrator's Console.
Under the global settings select the MTA tab.
Then there is a field called Maximum message size (kb). By default it is set to 10000 (10M). Change this value.